1. Technical Field
The present invention relates to a semiconductor circuit and a constant voltage regulator employing the same, and more particularly, to a semiconductor circuit for use in constant voltage regulation which can prevent variations in output voltage due to abrupt changes in input voltage, and a constant voltage regulator employing such a semiconductor circuit.
2. Description of the Background Art
Voltage regulators are employed in power supply circuitry which generates a regulated voltage from an input voltage to drive a load circuit that operates with constant power. In electronic applications, a voltage regulator is implemented in a single integrated circuit (IC), typically together with load circuitry, such as a microcontroller or other electronic components, to which electrical power is supplied from an external power source such as battery.
FIG. 1 is a circuit diagram schematically illustrating a configuration of a known voltage regulator 101.
As shown in FIG. 1, the voltage regulator 101 comprises a series regulator that converts an input voltage V111 supplied from a power supply terminal 111 to a regulated, constant output voltage V113 for output to an output terminal 113, consisting of a driver transistor M112, being a p-channel metal-oxide semiconductor (PMOS) device, having a source terminal thereof connected to the power supply terminal 111 and a drain terminal thereof connected to the output terminal 113; a pair of voltage divider resistors R111 and R112 connected in series between the output terminal 113 and a ground terminal 112 to form a feedback node therebetween; a reference voltage generator 116 connected between the input terminal 114 and the ground terminal 112; and a differential amplifier 115 having a non-inverting input thereof connected to the voltage divider node, an inverting input thereof connected to the reference voltage generator 116, and an output thereof connected to a gate terminal of the driver transistor M112, with a pair of power supply inputs thereof connected between the input terminal 114 and the ground terminal 112.
Components of the voltage regulator 101 may be integrated into a single IC, with the input voltage V111 being input from an external power source connected to the power supply terminal 111, and the output voltage V113 output to a load circuit connected to the output terminal 113.
During operation, the driver transistor M112 conducts an electric current therethrough according to a voltage applied to the gate terminal, so as to output a regulated output voltage V113 to the output terminal 113. The voltage divider resistors R111 and R112 generate a feedback voltage Vfb proportional to the output voltage V113 at the feedback node therebetween, whereas the reference voltage generator 116 generates a reference voltage Vref for comparison with the feedback voltage Vfb. The differential amplifier 115, receiving the feedback voltage Vfb at the non-inverting input and the reference voltage Vref at the inverting input, controls operation of the driver transistor M112 according to a result of comparison between the differential inputs Vfb and Vref, thereby regulating the output voltage V113 to a desired constant level.
FIGS. 2A and 2B are graphs showing the voltages V111 and V113 in volts (V) plotted against time in microseconds (μs), obtained at the power supply terminal 111 and the output terminal 113, respectively, during operation of the voltage regulator 101.
As shown in FIGS. 2A and 2B, the output voltage V113 of the voltage regulator 101, which is normally regulated to a constant level of approximately 3.3 V, experiences a sharp, transient change as the power supply voltage V111 suddenly changes in amplitude. Specifically, the output voltage V113 “overshoots” (i.e., rises sharply and transiently above the constant level) at time t0 where the power supply voltage V111 suddenly increases from 5 V to 25 V, and then “undershoots” (i.e., falls sharply and transiently below the constant level) at time t1 where the power supply voltage V111 suddenly decreases from 25 V to 5 V.
One problem encountered by the voltage regulator 101 depicted above is that those sharp transient changes of the output voltage V113, if significant, can adversely affect proper operation of the load circuit powered through the regulator circuitry. In practice, a large voltage overshoot of e.g., 1.0 V may damage the load circuit where the voltage V113 exceeds its rated maximum voltage, whereas a large voltage undershoot of e.g., 1.0 V may cause the load circuit to fail or malfunction where the voltage V113 exceeds its minimum operating voltage.
To counteract the problem, various methods have been proposed to provide a voltage regulation circuitry whose output voltage is stabilized against variations in input power supply voltage.
For example, one conventional method provides a voltage regulator formed of a differential amplifier circuit that outputs an output voltage to an output terminal connected with a transistor switch. According to this method, the voltage regulator is equipped with a voltage comparator that monitors the output voltage to control a gate voltage of the transistor switch according to a result of comparison between the output voltage and a reference voltage. Upon detecting a voltage overshoot due to a sudden change in input voltage, the voltage comparator causes the transistor switch to discharge capacitance, thereby stabilizing the output voltage.
One drawback of this method is that using the voltage monitor is costly since it includes a comparator adding to cost and power consumption in the voltage regulator. The method also has a drawback in that the feedback control based on the voltage comparator requires a certain period of time until the output voltage is adjusted in response to the feedback signal received, making the system less effective or practical than would be desired for its intended purpose.
Another conventional method provides a voltage regulator using an output transistor that regulates an output voltage according to a control signal output from an error amplifier comparing the output voltage against a reference voltage. According to this method, the voltage regulator is equipped with a voltage monitor consisting of a constant current circuit and a capacitor, which monitors a power supply voltage input to the voltage regulator and temporarily increases power supplied to the error amplifier upon detecting a sudden change in the power supply voltage. Increasing power input to the error amplifier enables the error amplifier to operate with a high slew rate, resulting in the control circuit exhibiting good response to the changing power supply voltage.
This method has a drawback in that, for proper functioning of the capacitor-based voltage monitor, the voltage regulator involves a capacitor of several picofarads, which is large in size and thus costly to implement on an IC-packaged device. Moreover, the method is not suitable for battery-powered applications, since supplying a large supply voltage to the error amplifier, if temporary, can reduce lifetime of the battery supplying power to the voltage regulator.